Why lakes are lowered in winter

The seasonal guide curve

Each Corps of Engineers reservoir is run under a water control manual. For many of them, the manual sets a target level that changes through the year, often called the guide curve or rule curve. The lake is held high through summer and lowered to a winter level in autumn, then allowed to refill in spring.

On this site, full pool for those lakes follows the curve: a lake page compares the reading with the full pool that applies on that day, and notes when it changes during the year. Of the 324 Corps lakes we track, 60 lowered full pool by at least a foot over the past year, and 17 by ten feet or more.

Why operators draw lakes down

Room for winter and spring floods

Across much of the East, South and Ohio Valley, the heaviest rain and snowmelt come between late fall and spring, when the ground is wet and plants use little water. A lower winter pool leaves more empty space in the reservoir to catch those floods before they reach the river below.

Releases with a purpose

The water let out during a drawdown is used. It can add flow for barge navigation and water quality downstream during dry autumn months, and it runs through turbines at dams with hydropower. Summersville Lake in West Virginia is a well-known example: its autumn drawdown releases are what make the Gauley River's fall whitewater season.

Shoreline and dam work

Low winter water exposes the shoreline, which gives the agency and lake residents a window to inspect and repair dams, docks, seawalls and boat ramps. In colder places, a lower level can also reduce ice damage along the shore.

Western reservoirs run by the Bureau of Reclamation follow a different rhythm: they usually fill with snowmelt in spring and early summer, then fall through the irrigation season into autumn. That decline is also normal for the time of year.

Largest seasonal drawdowns

The swing between the highest and lowest full pool over the past 365 days, from each lake's published curve. Recomputed with every update.

How to tell a drawdown from a low lake

Look at two numbers on the lake page, not one:

• Versus full pool tells you how far the lake is from today's target. During a drawdown that target is itself lower, so this number stays small when the drawdown is on schedule.
• For the date compares today with the same calendar date in up to 30 past years. If it says about normal, the lake is where it usually is at this time of year, however low it looks from the dock.

If a lake is well below its winter target and much lower than usual for the date, the cause is usually dry weather upstream or a change in releases. The ranking sorts every lake by the second number.

When do drawdowns start?

It varies by lake and by district, typically from September through November, with refilling beginning between late winter and spring. Each lake page's year chart draws the full pool line for the past 365 days, so you can see exactly when that lake's target dropped and when it rose again. Dates can shift in a given year when weather, drought plans or dam work call for it, and the agency announces those changes on its own lake pages.

More lake level guides

• What “full pool” means: Full pool, flood pool and dead pool: the layers of a reservoir, and why a lake above full pool is not a full lake.
• How to read a lake elevation: What a reading like 1,071.3 ft means, why lakes cannot be compared by elevation, and what NGVD29 and NAVD88 are.
• Will my dock or ramp be dry?: A tape-measure method to find the elevation of your dock or ramp, and what to watch for when the water is low.
• Who decides how full a lake is: How the Corps of Engineers and Reclamation decide how much water to hold or release, and why a lake can fall after rain.